1. Field of the Invention
The invention relates to a method for rolling a plate and a rolling mill using the method, and further to e roll to be used for such method and rolling mill, all of which utilize a roll shifting operation in which upper and lower work rolls are axially shifted in opposite directions to thereby roll a plate, in combination of roll bending operation in which a force is applied to bend the upper and lower work rolls to thereby roll a plate.
2. Description of the Related Art
Japanese Patent Publication No. 63-62,283 and Japanese Unexamined Patent Public Disclosure No. 1-266,902 have suggested a rolling mill which controls a cross-section of a plate to be rolled across its width to thereby provide the plate with a flat surface. As illustrated in FIG. 1 (Prior Art), the roll shifting type rolling mill is adapted to roll a plate 2 with upper and lower work rolls 1 that are axially shifted in opposite directions indicated by arrows A and B.
These conventional roll shifting type rolling mills have the following problems. In these conventional roll shifting type rolling mills, the upper and lower work rolls 1 are provided with an initial crown in order to apply the crown control effect to all plates ranging in width from wide to narrow. Thus, if the crown control effect is to be enhanced for a plate having a narrow or intermediate width, the upper and lower work rolls 1 have to be enlarged in parallel. Such an enlargement of the work rolls 1 is accompanied by excessive difference in roll diameter of the upper and lower work rolls in the axial direction, which in turn causes an excessive difference in both peripheral speed and surface pressure of the work rolls, resulting in oscillation and/or damage to a plate to be rolled.
Crown control by means of a roll bending mechanism or apparatus as illustrated in FIG. 2 (Prior Art) can provide only small control effects to a plate having an intermediate or narrow width due to the characteristics of the deflection curve of the roll.
Thus, the above mentioned conventional rolling mills have a problem in that they can provide only a small crown control effect to a plate having an intermediate or narrow width. In other words, as illustrated in FIG. 3A, the plate crown control effect caused by roll shift operation is relatively large in a plate having a wide width, and relatively small in a plate having a width ranging from intermediate to narrow. In addition, as illustrated in FIG. 3B, in the roll bend apparatus illustrated in FIG. 2, the plate crown control effect caused by a roll bend operation is relatively large in a plate having a wide width, while relatively small in a plate having an intermediate to narrow width. Hence, as illustrated in FIG. 3, even if the crown control effect caused by a roll shift operation is combined with the crown control effect caused by a roll bend operation, the combined crown control effect is relatively small in a plate having an intermediate to narrow width, while excessive in a plate having a wide width.
In addition, as illustrated in FIG. 4, if the curvature of a middle portion of rolls 1 is considerably changed in order to apply a larger crown control effect to a plate to be rolled having an intermediate to narrow width by using a conventional roll shift operation, the difference in a roll diameter between larger diameter portions indicated as D3 and D4 and smaller diameter portions indicated as D1 and D2 becomes larger with the result that the pressure at which the rolls 1 are in contact with back-up rolls 6 becomes excessive thereby possibly causing rolling defects.
Thus, the inventors have invented a roll for use with a rolling mill and have filed with Japan Patent Office, on Feb. 25, 1994, Japanese Patent Application No. 6-27085, which is not prior art to the present invention. In this rolling mill, a plate to be rolled is kept inclined during rolling to thereby provide larger variability of the curvature of the external surface of the roll barrel, larger crown control and less oscillation of the plate to be rolled.
As illustrated in FIG. 5, the above-mentioned roll has a single straight region 3 located at the center of a roll barrel, auxiliary crown control regions 4 located at the opposite ends of the roll barrel, and primary control regions 5 located between the straight region 3 and the auxiliary crown control regions 4. The roll has a bus comprising a straight line inclined to a long axis of the roll barrel in the straight region 3, steep convex or concave curvatures in the primary crown control regions 5, and gentle convex or concave curvatures in the auxiliary crown control regions 4. In addition, the roll is designed to have opposite ends having an equal diameter. The rolling mill disclosed in Japanese Patent Application No. 6-27085 surely makes it possible to enhance the crown control effect for a plate having an intermediate or narrow width and to further prevent the above-mentioned excessive difference in roll diameter. However, the rolling mill in question may be accompanied by irregularities in distribution of plate width of wide width plates with the result that the distribution of widths is not smooth.
It is therefore an object of the present invention to solve the above mentioned problems. Specifically, one of objects of the invention is to provide a method for rolling a plate and a rolling mill which imparts enhanced crown control effect to a plate with a small difference in a roll diameter, and capable of use with intermediate or narrow widths, and which provides a smooth distribution of plate width even in plates having wide widths.
A variety of rolling mills have been suggested for flattening a rolled plate by controlling cross-sectional shape of a plate to be rolled in the widthwise direction of such a plate. One of such rolling mills is known as a roll shift type rolling mill. For instance, Japanese Unexamined Patent Public Disclosure No. 1-266902 has suggested a roll shift type rolling mill, as illustrated in FIG. 6A, which shifts a pair of upper and lower work rolls 1 in opposite axial directions to thereby roll a plate 2 with the upper and lower work rolls 1 being supported by back-up rolls 6.
The upper and lower work rolls 1 in this rolling mill are designed to have an initial crown so that the upper and lower work rolls 1 have curved contours which are complementary to each other, in order to provide greater plate crown control effect to the plate 2 to be rolled. Thus, depending on the direction in which the upper and lower work rolls 1 are shifted, the plate crown applied to the plate 2 is a concave crown as illustrated in FIG. 6B or the convex crown as illustrated in FIG. 6C.
The rolling mill can vary the plate crown by shifting the work rolls 1 to thereby widen the controllable range. However, in actual rolling, there are several factors which may deteriorate rolling performance (such factors will be discussed with reference to FIG. 15A, 15B and 15C). The rolling mill has no countermeasures against such factors. One of such factors is that, as illustrated in the left figure of FIG. 15C, the surface pressure at roll ends is increased by the roll shift operation with the result that it is not possible to set the plate crown to be optimum.
In addition to the above mentioned factor, the factors which deteriorate rolling performance include a heat crown effect, as illustrated in FIG. 15A, which is caused by heat applied to the rolls, and deflection of the rolls caused by rolling load applied to the rolls as illustrated in FIG. 15B. In actual rolling, these factors affect the rolling alone or in combination, whereby it is not possible to set the plate crown to be optimum.
In view of the foregoing problems of the prior art, another object of the present invention is to provide a roll to be used with a roll shift operation, which compensates for factors which deteriorate rolling performance. It is a further object to provide a roll which provides a roll crown to deal with every rolling condition, and also to provide a roll shift type rolling mill using such a roll.